With Zend Framework the class definition of the Zend_Exception takes care of the php version and so php < 5.3 triggers an Fatal Error beacause both defnitions are in the same file.
Please have a look at ​ZF-8984

Hello, I have experienced a problem where xcache loads a class incorrectly before a validating if statement which would otherwise return/exit the script. Wrapping the class in else brackets resolves this issue, however I have seen this syntax in use in a number of Wordpress plugins which might mean its worth looking into this.